Among the various molecules that regulate T cell function, Lymphocyte Activation Gene 3 (LAG-3) has garnered significant recent interest. LAG-3 is a CD4 homolog originally cloned in 1990. The gene for LAG-3 lies adjacent to the gene for CD4 on human chromosome 12 (12p13) and is approximately 20% identical to the CD4 gene. LAG-3 is expressed on B cells, NK cells, tumor infiltrating lymphocytes (TIL), and a subset of dendritic cells. Recently, we showed that LAG-3 was relatively over-expressed on HA- specific transgenic T cells rendered anergic in vivo by encounter with cognate self- antigen. In this system, tolerized T cells display regulatory function both in vitro and in vivo;and regulatory activity could be functionally blocked with a LAG-3 specific monoclonal antibody. By transducing naive CD4 T cells with a full length LAG-3 construct, we could confer in vitro regulatory activity. Studies in humans have demonstrated increased LAG-3 expression on tumor-infiltrating FoxP3+ regulatory T cells. In addition to CD4 T cells, activated CD8 T cells increase LAG-3 expression in response to antigen stimulation, and LAG-3 positive CD8 T cells frequently infiltrate tumors in cancer patients. Observations using LAG-3 knockout mice demonstrate that CD8 T cells undergo enhanced homeostatic proliferation in vivo if LAG-3 is absent. Studies using human PBMC also suggest a negative regulatory role for LAG-3 on CD8 T cell expansion, as LAG-3 antibody blockade combined with superantigen stimulation results in increased proliferation CD8 T cells compared to superantigen alone. We have generated significant preliminary and published data suggesting that LAG-3 blockade can alter the immune response to both autochthonous and implanted tumors and suggest that LAG-3 blockade may eventually play a significant role in tumor immunotherapy. PUBLIC HEALTH RELEVANCE: LAG-3 is a molecule on the surface of some white blood cells known as T cells;T cells are responsible for fighting infection, and in some cases can help to eliminate tumors. Since LAG-3 on T cells inhibits their function, we propose to improve the immune response to cancer by blocking LAG-3 with small proteins known as monoclonal antibodies. The overall goal of these studies is to understand how LAG-3 works, and then to use this information to target LAG-3 in an effort to improve immune treatments for cancer patients.